1. Field of the Invention
This invention relates to laser range finders and in particular to a multiple wave length laser source for use therein. Laser rangefinders exist that operate in both the 3-5 .mu.m atmospheric window (DF at 3.8 .mu.m) and the 8-13 .mu.m window (CO.sub.2 at 10.6 .mu.m). At sea level, variations in the attenuation of radiation in the 8-13 .mu.m band are due to variations in the amount of atmospheric water vapor, because of a water vapor continuum absorption located in this spectral region. Variations in the attenuation of the 3-5 .mu.m band are primarily due to changes in the aerosol concentration in the atmosphere i.e. visibility changes; the longer wavelengths of the 8-13 .mu.m band are less affected by aerosol scattering losses.
A major design consideration for a laser radar is the selection of the operating wavelength. Both the CO.sub.2 and DF laser wavelengths lie in the "eye-safe" region, so that differences in their atmospheric propagation properties under envisioned operating conditions of the laser rangefinder should decide the optimum operating wavelength. For maximum flexibility, both laser wavelengths should be incorporated into the transmitter. If two independent wavelength laser devices are used, however, problems in optical alignment of components which can be opaque to visible radiation are encountered. Furthermore, complex beam-mixing optics would not be as rugged as a single cavity system under field conditions. It is evident that a considerable reduction in system complexity is possible if the two lasers can be operated in the same optical cavity.
2. Description of the Prior Art
In U.S. Pat. No. 3,548,212 which issued Dec. 15, 1970, Whetter describes a multi beam laser tracking system in which laser beams of different wavelengths and from different optical cavities are employed. The lasers appear to share a common back mirror, although they use separate portions of that mirror, while the output mirrors are the partially reflecting ends of the crystals. It is noted that the laser beams are not combined into a common beam, but instead are directed forward to a common target at a known distance from the source.
In U.S. Pat. Nos. 3,437,820 which issued Apr. 8, 1969 to Thompson et al and 3,549,256 which issued Dec. 22, 1970 to Brienza et al, optical rangefinders are described in which a single output beam is produced by combining the two beams from different optical cavities with a dichroic mirror. Such devices require careful alignment of steering mirrors to ensure that the direction of propagation of the two different wavelength laser beams is the same.